Stirred tank reactors (STR) form an integral component of chemical, pharmaceutical, and the fermentation industries. These types of reactors have been in operation for last several decades and a number of investigators have analyzed them in detail to optimize the designs based on the power consumption, mass and heat transfer, and the internal hydrodynamics. In the stirred reactors, energy is supplied in the form of a kinetic energy by rotating the impeller at desired speed. STRs have largely been used for (i) mixing or blending of two miscible liquids, (ii) generation of dispersions for gas-liquid and liquid-liquid reactions, (iii) keeping the solid particles in suspension to facilitate the solid fluid contact to achieve solid dissolution, (iv) crystallization, etc. The energy requirement of these processes forms a significant part of the total energy and contributes toward major expenses. Thus, the efficiency of a stirred tank reactor mainly depends on the impeller design and its location in the stirred reactor.
U.S. Pat. No. RE42882 provides a method and apparatus for rapid and homogeneous mixing or reacting a fluid mixture, wherein two or more independent and offset fluid transporting fractals allow the scaling and intermingling of two or more fluids separately and simultaneously prior to contacting the fluids with one another, the geometry of one fractal is different from the geometry of a second fractal. Fractals are constructed using an initiator structure, or parent structure, with self-similar structure added at smaller and smaller scales in the form of an “H”. Furthermore, one of the fractals is bifurcated at an angle between perpendicular and parallel to a flow direction of the inlet of that fractal. However turbulence inducing mechanical mixing devices, such as impellers, blenders, and impinging devices, is not used in U.S. Pat. No. RE42882.
International Publication No. WO9948599 provides fractal structures arranged to minimize the intersection of recursive fluid flow paths which comprises an improved fluid transporting fractal. A notable feature of the structures of this invention is the positioning of fractal stages along the direction of flow wherein stages of either progressively smaller or progressively larger scales are arranged serially in the direction of flow that lower the turbulence.
Mike Kearney in Chem. Eng. Comm. Vol. (1), 1999 describes applications of engineered fluid transporting fractals which include use as alternatives to turbulence, controlled formation of fluid geometry, broad range of fluid control and rapid transition of effective fluid dimension.
Further. Patrick Spicer in the Journal of Colloid and Interference Science 184, 112-122 (1996) 0601 discloses the effect of impeller type and shear rate on the evolution of floe size and structure during shear-induced flocculation of polystyrene particles with aluminum sulfate which is investigated by image analysis. The concepts of fractal geometry are used to characterize the floc structure.
Additionally, Joel J. Ducoste, in AIChE 43 (2), 328-338, 1997 describes effects of tank size and impeller type for STRs, wherein turbulence intensity increases with increasing tank size regardless of impeller type. U.S. Pat. Pub. No. 2007/0299292 and U.S. Pat. Pub. No. 2010/0307665 describe different fractal patterns for STRs.
Typically (except for the highly viscous fluids), the system operates in turbulent regime. Usually, the distribution of energy dissipation is considerably heterogeneous. Thus, for instance, for a paddle mixer, 90% of the input energy is dissipated below the impeller while the remaining 10% is dissipated above the impeller. Also, for a pitched blade down flow turbine (PBTD), 30% energy is dissipated in the impeller region, 57% below the impeller and just 13% above the impeller. Usually, the impeller region is the most active zone of the reactor and also a region yielding high transient shear gradients. Thus, uniform spatial distribution of energy is difficult to achieve in the conventional STRs. Also, for achieving uniform temperature throughout the reactor while operating it at lower impeller speed to avoid high shear zones (mainly for shear sensitive media), the conventional impellers may not be applicable.
Therefore, there is a need in the art to look for alternatives that would make the entire reactor active in a hydro-dynamically similar manner. Thus, it is an objective of the invention to provide an efficient impeller to achieve uniformity throughout the stirred tank that can yield better mixing and low shear at relatively low power consumption.